Electrostatic ion sprays have been utilized in the past for increasing the efficiency of various types of heat exchanger apparatus either for the purpose of cooling the apparatus or for the purpose of increasing efficiency. These types of systems are illustrated in U.S. Pat. Nos. 3,872,917 issued Mar. 25, 1975; 3,794,111 issued in Feb. 26, 1974; 3,862,391 issued Jan. 21, 1975; 3,757,079 issued Sept. 4, 1973; 3,735,175 issued May 22, 1973; 3,747,284 issued July 24, 1973; and 3,670,606 issued June 20, 1972, to Oscar C. Bloomgren, Sr., et al.
One of the problems in applying this technology to high temperature solar energy receivers is the problem of arcing between the electrodes and the heat exchanger used in the solar energy receiver when temperatures of, for instance, 1500.degree. F. are exceeded. In order to achieve ion spray efficiency increases at 1500.degree. F. and above it would be necessary to provide complicated control circuitry for sensing the condition preceeding arcing, and preventing the arcing by reducing the electrostatic voltage. The difficulty in such a control system centers around the non-uniformity of the flow of the heat transfer fluid, impurities in the heat transfer fluid, and the unpredictable nature of the heat transfer fluid at high temperatures.
As a solution to the above mentioned problem, a high intensity radio frequency (RF) field is generated instead of the electrostatic (D.C.) field. The term RF is used herein to include electromagnetic energy in a band between 1 KHz and 10.sup.5 MHz although this invention is not limited to this frequency band. In one embodiment of this invention an RF electrode or probe is placed in the vicinity of the heat exchanger utilized in the solar receiver and a dielectric shield in the form of a plate or other member is interposed between the electrode and the heat transfer fluid. When used with solar receivers having an impingement cooled window, this window can double as the dielectric shield. Because RF energy is utilized, an intense RF field penetrates through the dielectric shield to the fluid side of the dielectric barrier to either produce an ion spray or is utilized directly in breaking up the thin film boundary layer between the heat exchanger walls and the fluid in the heat exchanger which increases heat exchanger efficiency. It will be appreciated that if the dielectric shield were utilized with an electrostatic field, there would be no energy propagation through the dielectric shield. It will be further appreciated that the higher the frequency, the more transparent will be the dielectric shield to RF field penetration. The dielectric shield prevents arcing by preventing the possibility of establishing a single high-conductivity path through the fluid. In one embodiment the dielectric shield or barrier is transparent to light and serves as a window through which focused solar energy passes on its way to the heat exchanger.
It will be noted that in the prior art 60 cycle A.C. fields are used for enhancement of heat exchanger efficiency. However in these systems the heat exchanger is connected to one side of the A.C. generator and there is no net ion propagation towards the heat exchanger, but rather an oscillating field is established between the electrode and the heat exchanger. The subject system on the other hand eliminates a high conductivity return from the heat exchanger to the power source so that a net propagation direction is established.
One type of solar energy receiver to which this technique is especially applicable is described in a U.S. patent application entitled Solar Energy Conversion System, Ser. No. 612,434 filed Sept. 11, 1975 by Philip O. Jarvinen and assigned to assignee hereof. In this solar energy receiver the heat transfer fluid is a gas which is introduced between the heat exchanger and a quartz window from which point it passes through the heat exchanger. In one embodiment the heat exchanger is a silicon carbide honeycomb structure with channels running parallel to the optical axis of the system. The gas in this embodiment may be air and the exchange of heat from the heat exchanger to the air flowing through it is enhanced by virtue of the RF-generated ion spray which, according to one interpretation, breaks down a boundary layer between the heat exchanger walls and the gas flowing through the heat exchanger. While the subject invention is not limited to any one theory of operation, it is thought that the ion stream may produce acoustic shock waves which break up this layer, especially when the RF source is pulsed, and in one embodiment, a Tesla coil is used to achieve a high intensity pulsed RF field which propagates towards the heat exchanger. As is well known, a Tesla coil produces a broad spectrum of RF energy.
It is also possible to achieve increased heat exchanger efficiency with RF signals at a single frequency and high power microwave generators may be employed for this purpose. For microwave frequencies it is thought that the RF field may act directly on the boundary layer to break it up although ions may also be produced in the process.
Insofar as the receiver is concerned, solar energy is focused onto its heat exchanger through a transparent window which causes the temperature of the heat exchanger to rise. Typically the heat exchanger's temperature rises to between 1,500.degree. F. and 2,500.degree. F. In order to contain this heat, the heat exchanger is thermally insulated from the body of the receiver. Any energy radiated by the hot heat exchanger and not contained by the insulating material is reflected back into the heat exchanger by the walls of the receiver and the impingement cooled window, such that the heat exchanger acts like a black body receiver to trap all incoming solar energy.
The subject invention is not, however, limited to a solar energy receiver of the type described and in general the invention relates to the increase in efficiency of any heat exchanger by the provision of RF energy and/or RF-generated ions at the heat exchanger conduit which channels the fluid through the heat exchanger so that the transfer of energy from the walls of the heat exchanger to the fluid will be enhanced.
In an alternative embodiment, the Tesla coil RF energy source may be replaced with a microwave generator such as a magnetron, and a number of RF electrode configurations are provided for injecting the microwave energy. These configurations include coaxial cables with sharpened central conductors as electrodes, and waveguides with reentrant cones which permit the passage of sunlight to the heat exchanger. If waveguides having portions which permit entry of solar energy are used, they may be placed in front of the solar receiver without blocking the focused solar energy.
It is therefore an object of this invention to provide an improved solar energy receiver utilizing an RF field for the enhancement of heat exchange within the receiver.
It is another object of this inventions to provide ion spray efficiency increases in a high temperature environment by the utilization of RF energy and a dielectric shield to prevent arcing in which the shield is positioned between the electrode at which the high intensity RF field is generated and the heat transfer fluid.
These and other objects of the inventions will be better understood where in connection the following specification in conjunction with the following drawings wherein